Synergistic combination of silicates and barium salts for inhibiting the attack of alkaline solutions on aluminum containing materials

ABSTRACT

MATERIALS WHICH ARE PRINCIPALLY COMPRISED OF ALUMINUM, AND WHICH ARE SENSITIVE TO THE ATTACK OF ALKALINE SOLUTIONS, ARE PROTECTED AGAINST SUCH ATTACK BY A SYNERGISTIC COMBINATION OF WATER SOLUBLE SILICATES AND BARIUM SALTS, WITH OR WITHOUT WELL-KNOWN SURFACE ACTIVE AGENTS AND SOLVENTS.

United States Patent US. Cl. 252-387 1 Claim ABSTRACT OF THE DISCLOSURE Materials which are principally comprised of aluminum, and which are sensitive to the attack of alkaline solutions, are protected against such attack by a synergistic combination of water soluble silicates and barium salts, with or without well-known surface active agents and solvents.

This invention relates to the protection of alkaline sensitive aluminum-containing substrates from the attack of alkaline solutions, particularly solutions of sodium and potassium hydroxides. More particularly, this invention relates to the protection of alkaline sensitive substrates which are principally comprised of aluminum from the attack of alkaline solutions by a synergistic combination of water-soluble silicates and barium salts.

Caustic and other alkaline solutions are highly corrosive to many surfaces, causing extreme pitting and dissolution of the surface of sensitive materials. Corrosion of surfaces by alkaline materials presents a serious limitation with respect to cleaning of such surfaces. For example, highly alkaline solutions have proved very effective for removing such soils as baked-on food soils, oleoresinous films, fatty soils, oxidized hydrocarbons, waxy soils, carbonaceous soils and the like which are difficult to remove without using caustic or other highly alkaline solutions. The sensitivity of materials to alkaline attack presents problems in a wide spectrum of applications including the metal working industry, cleaning equipment in food processing installations, maintenance cleaning of transportation vehicles, dish washing and paint stripping operations.

Current methods for cleaning alkali sensitive materials have generally avoided the use of caustic or strong alkaline solutions in cleaning processes because of the disadvantages noted above. Solvents or emulsions have found limited use as cleaners. However, they generally are not effective for many soils; they do not produce surfaces suitable for subsequent finishing operations; and they present problems of toxicity and flammability. A great deal of research has been directed to the development of neutral or mildly alkaline solutions which offer detergent action. However, this detergent action is usually based on surfactants alone or in combination with sodium borate, which have proved ineffective for the more tenaciously held soils and are practical only for light duty cleaning operations.

Sodium silicate-containing alkaline cleaners have probably been the most widely accepted materials for cleaning alkaline sensitive substrates such as aluminum and its alloys; but such cleaners suffer from several limitations of which the most serious is the restriction on level of alkalinity. Inhibition of alkaline solution attack by sodium silicates appears to be effective only when the ratio of SiO /Na O is greater than 1. The inhibiting effect occurs only at certain concentrations of sodium metasilicate when the ratio is equal to 1. Therefore, the high alkalinity necessary for the removal of many soils cannot be used with- 3,655,582 Patented Apr. 11, 1972 out causing attack of the substrate because ratios less than one are ineffective. Furthermore, to achieve adequate cleaning it has been found necessary to utilize long soaking periods or mechanical action to accomplish release of the soil. However, we have discovered that by adding a water-soluble barium salt to the sodium silicate-containing alkaline cleaners a surprising synergistic effect results, and very satisfactory corrosion inhibition takes place without these objectionable limitations. The same results do not occur when the barium salt is added but the sodium silicate is omitted. The presence of both is essential.

Substrates which are particularly sensitive to attack or corrosion by alkaline solutions primarily consist of metals which form soluble reaction products with strongly alkaline solutions, aluminum and its various alloys being the more important examples. The protection of aluminum is extremely important to industry because of its extensive use in a wide variety of applications, and the very rapid attack of alkaline solutions thereon. Alloys and other compositions containing major amounts of aluminum are also susceptible to alkaline attack and are included within the substrates which are benefitted by the present invention.

It is an object of the present invention to clean alkaline sensitive materials with alkaline detergents without deleterious corrosion.

It is a further object of the present invention to inhibit the attack of very strong alkaline cleaners on aluminum and its alloys, removing tenacious soils and coatings without damage to the substrate.

It is a further object of the present invention to permit the use of aluminum and its alloys in applications where such metals could not previously be used because of the necessity for strong alkaline cleaning.

It is a further object of the present invention to provide a means for controlling the rate of the caustic attack on aluminum and its alloys during etching and chemical milling operations.

We have discovered that the attack of strong alkaline solutions, particularly those containing caustic, on alkaline sensitive metals such as aluminum and its alloys is substantially reduced by the synergistic combination of a water-soluble silicate and a water-soluble barium salt as indicated above. It is significant to note that either component alone produces only a minor reduction in the corrosion rate. as seen in Table I below. However. with both the silicate and barium salt components present (at least 001 Weight percent of the barium salt and at least 0.02 weight percent of the silicate) a marked reduction in corrosion rate is observed.

The practice of this invention need not be restricted to the use of BaC1 as other water soluble barium salts. such as the hydroxide, oxide, or silicate, could be used in place of the chloride. Similarly, other silicates may be used in place of the sodium metasilicate, examples being lower alkalinity sodium silicates or potassium silicate.

Further indications of the unexpected and unobvious results obtained by the synergistic combination of the present invention are evident in the fact that salts of other alkaline earth metals, Ca or Sr, cannot be used as they are precipitated as the silicate salts. All other cations either are not effective inhibitors with silicate (Na, K, Li, for example), or produce insoluble silicate salts. Anions other than the silicate ion are not effective with barium ion, as they either produce insoluble precipitates with barium (cg. carbonate, phosphate or sulfate), remove the barium ion by sequestering (e.g. polyphosphates), or are simply ineffective {c.g. fluoride).

The synergistic combination of the present invention optionally may be utilized with solvents and/or any of a number of conventional surface active agents that have EXAMPLE 1 The data in Table I were obtained by immersing aluminum foil samples in a solution of 0.4% NaOH at 60 C., and after 16 hours the samples were removed, rinsed, dried and weighed. From the weight loss, time and dimensions of the test pieces before and after immersion, the corrosion rate is calculated in mils per year. As the data show, the addition of 0.1% Na metasilicate and 0.1% BaCl reduce the corrosion rate of aluminum from 30,000 mils per year to 0.3 mil per year.

TA 13 LE I Corrosion rate in mils per year 1 Percent Na Percent metaslllcate BaClz 0.4% NaOH 4.0% NaO H 1 Aluminum sample N0. 3, 60 C.

EXAMPLE 2 (a) In a manner similar to the procedure set forth in Example 1, 0.1% Na metasilicate and 0.1% Ba (OH) are employed instead of the sodium metasilicate-barium chloride combination.

(b) Likewise, an additive of 0.1% Na metasilicate and 0.1% barium oxide are used instead of the sodium metasilicate-barium chloride combination employed in Example 1.

Similar reductions of the corrosion rate, as were ob tained in Example 1, are obtained with both additive combinations (a) and (b).

4 EXAMPLE 3 (a) In a manner similar to the procedure set forth in Example 1, 0.1% Na orthosilicate is substituted for the metasilicate.

(b) Likewise, Na silicate (3.2 siO /Na O ratio) is used as the additive in lieu of the metasilicate in Example 1.

(c) Likewise, an additive is used as in Example 1 but Na silicate (2.6 SiO /Na O ratio) is substituted for the metasilicate.

(d) Similarly, an additive is used as in Example 1 but potssium silicate (2.1 SiO /K O ratio) is substituted for the metasilicate.

In each of these four examples similar corrosion rate reductions, as were obtained in Example 1, are obtained.

We claim:

1. In a process of treating with an aqueous solution of sodium or potassium hydroxide a material made of aluminum, or an alloy thereof whose principal component is aluminum, the improvement of mixing with said aqueous solution an effective amount of a synergistically operative corrosion-inhibiting combination comprising a water soluble silicate selected from the group consisting of sodium silicate, sodium orthosilicate, sodium metasilicate, potassium silicate, potassium orthosilicate, and potassium metasilicate and a water-soluble barium compound selected from the group consisting of barium chloride, barium hydroxide, barium oxide, and barium silicate, the silicate being present in an amount equal to at least 0.02 weight percent and the barium salt being present in an amount equal to at least 0.01 weight percent.

References Cited UNITED STATES PATENTS 1,912,175 5/1930 Blough et al. 252387 2,425,907 8/1947 Wegst et al 252l56 2,738,294 3/1956 Spence 252l56 2,882,135 4/1959 Elliott 252l56 FOREIGN PATENTS 1,160,267 12/1963 Germany 252387 OTHER REFERENCES Mears: Aluminum Chemical Plant," Chemical Trade Journal and Chemical Engineer, 1943, pp. 93-94.

RICHARD D. LOVERING, Primary Examiner I. GLUCK, Assistant Examiner US. Cl. X.R.

2l-2.7; 23l84; l34--2; 25279.5, 80, 156, 

